I was told by a nonprofit organization that I would receive a job offer letter, as long as I signed a contract first. In it, I would agree to work outside of my job description, work beyond my scheduled work hours, and help out at all company events, regardless of whether they have anything to do with my position. It said I should expect to work especially hard during my probationary period, as I would be learning all aspects of the organization.

These are things that I would expect to do for a nonprofit with a small staff. I don't understand the purpose of this document - especially since I would be an exempt employee. Is this standard, and I've just never come across it before?